Glycols and in particular ethylene glycol and propylene glycol are valuable materials with a multitude of commercial applications, e.g. as heat transfer media, antifreeze, and precursors to polymers, such as PET. Most glycols are prepared by industrial routes from petrochemicals derived from crude oil. For example, ethylene and propylene glycols are typically made on an industrial scale by hydrolysis of the corresponding alkylene oxides, which are the oxidation products of ethylene and propylene, produced from fossil fuels.
In recent years, increased efforts have focused on producing chemicals, including glycols, from renewable feedstocks, such as sugar-based materials. For example, US20110312050 describes a continuous process for the catalytic generation of polyols from cellulose, in which the cellulose is contacted with hydrogen, water and a catalyst to generate an effluent stream comprising at least one polyol.
CN102643165 is directed to a catalytic process for reacting saccharides in an aqueous solution with hydrogen in the presence of a catalyst in order to generate polyols.
As with many chemical processes, the reaction product stream in these reactions comprises a number of desired materials, diluents, by-products and other undesirable materials. In order to provide a high value process, the desirable product or products must be obtainable from the reaction product stream in high purity with a high percentage recovery of each product and with as low as possible use of energy and complex equipment.
In known processes to make glycols, the glycols are usually present at high dilution in a solvent, typically water. The water is usually removed from the glycols by distillation. Subsequent purification of the glycols is then carried out by fractional distillation. This process can have high costs both in terms of capital and operational expenditure. Further, repeated heating or maintenance at raised temperatures in the fractional distillation steps may also lead to decomposition of the desired glycol products.
When glycols are produced by hydrogenolysis of saccharides, a mixture of diols, including glycols and other by-products is produced. The main glycol constituents in the reaction product stream are monoethylene glycol (MEG), monopropylene glycol (MPG) and 1,2-butanediol (1,2-BDO). Other diols, such as 2,3-butanediol (2,3-BDO), pentanediols, hexanediols and heptanediols may also be present. The separation of these diols by fractional distillation is complicated due to the similarity in boiling points. For example, MEG and 1,2-BDO have normal boiling points of 198 and 196.8° C., respectively. Further, the isolation of a pure MEG overheads stream by fractional distillation from a mixture comprising MEG and 1,2-BDO is made impossible by the formation of a homogeneous minimum boiling azeotrope between MEG and 1,2-BDO at atmospheric pressure. A similar close-boiling, azeotrope-forming glycol pair is MPG and 2,3-pentanediol. Other close boiling and/or azeotropic mixtures may also be formed between other diols present, further complicating the purification process.
Degradation of the products at high temperatures makes the use of higher than atmospheric pressure for distillation undesirable.
Methods to separate diols and, in particular, 1,2-BDO and MEG have been described in the art.
U.S. Pat. No. 4,966,658 is directed to the separation of a mixture of 1,2-BDO and MEG using a process known as azeotropic distillation in which an azeotrope-forming agent is added to the mixture before distillation in order to facilitate separation. Suitable azeotrope-forming agents are stated to include 3-heptanone, o-xylene, cumene and heptane. A similar process is described in U.S. Pat. No. 5,423,955 for the separation of 1,2-BDO and MPG, in this case using (among others) toluene, o-xylene, cumene and heptane as azeotrope-forming agents. Azeotropic distillation can lead to an increase in relative volatility between the components but also leads to further process steps in order to remove the azeotrope forming agents.
CN102372600 describes an extractive distillation process for the separation of glycols. In this process, a mixture of MEG, MPG and 1,2-BDO are fed to a distillation column and contacted therein with an extractant. The top product, comprising the light extractant and 1,2-BDO, is then separated in a further distillation column. The bottom product, comprising MEG, MPG and extractant is subjected to further distillation to provide MEG as the bottoms product. Suitable extractants are stated to include C6-C9 aromatics, alkanes, alkenes, C6-C11 ketones or ethers with toluene, o-xylene, cumener, n-heptane, n-octane, 3-heptanone and diethylene glycol dimethyl ether mentioned as preferred extractant. This teaching appears to be somewhat inconsistent with the above cited cases, which name the materials as azeotrope-forming agents.
WO2015150520 discloses a process for separating monoethylene glycol from a mixture comprising monoethylene glycol and 1,2-butanediol, using a two column, pressure-swing distillation set-up.
It would be advantageous to provide a simple and efficient method suitable for the recovery of desired diol products, such as MEG or MPG, from a mixture of diols from a product stream derived from a saccharide hydrolysis process or other bio-based processes.